Although FRP has been used in various fields, as a method of producing an FRP structural material, general is a so-called “prepreg/autoclave molding” method wherein, after a preform having a shape of a structural material to be molded is formed beforehand by prepregs, the preform is cured in an autoclave set at a predetermined condition in temperature and pressure. However, recently a method of RTM molding reduces production costs, and that method is developed gradually. Many methods are proposed as RTM molding methods of producing panels and beam materials which are structural members for airplanes or architecture requiring high strength, lightweight and low cost or of producing FRP molded products such as outer panels of vehicles. For example, there are a methods of RTM molding for molding a large FRP structural material (for example, JP-A 12-145042) and a method of RTM molding using a resin distribution medium (for example, U.S. Pat. No. 5,052,906).
In the method of RTM molding disclosed in JP-A 12-145042, peel plys/resin distribution media are disposed on both surfaces of a reinforcing fiber substrate comprising a laminate of reinforcing fiber materials, the whole thereof is covered with a bag material, and a resin injection gate and an evacuation gate for reducing a pressure are provided relative to the inside covered with the bag material. In this state, under a condition of a room temperature or a heated atmosphere, a resin is injected from the resin injection gate while being reduced in pressure by evacuating the inside of the bag material through the evacuation gate. The resin flows from the upper surface side to the lower surface side of the reinforcing fiber substrate or from the lower surface side to the upper surface side to impregnate the resin into the reinforcing fiber substrate. After completion of impregnation, the resin is cured at room temperature or under a heated atmosphere condition and, after curing, the molded product is taken out from the mold by removing the bag material.
As the problem in the above-described molding method, although resin distribution media are disposed on both surfaces of the reinforcing fiber substrate, because the resin impregnation is basically carried out from one surface side relative to the reinforcing fiber substrate, there is a limit in the distance capable of being impregnated in the thickness direction of the substrate. Therefore, if the reinforcing fiber substrate is too thick, a predetermined impregnation becomes impossible.
It is known that permeability of resin into the reinforcing fiber substrate can be determined generally by the following equation.I=(ε/(1−ε))√(αP/2)×∫[dt√(μ(t)t)]                I: permeability, ε: resistance of substrate, α: constant,        P: vacuum pressure in substrate, μ(t): viscosity,        t: expiration time        
Permeability corresponds to a distance (thickness) of resin impregnation into the substrate.
With respect to resin impregnation into a reinforcing fiber substrate, although the constant and viscosity in the above-described equation are different depending on the kinds of the substrate and the resin, because the impregnation distance is astringent accompanying with expiration of time and, further, there occurs an increase of the viscosity and the resin gradually becomes a gel, a limit is generated in the distance in which the resin can impregnate and, if the thickness of the reinforcing fiber substrate is a certain thickness or more, it becomes impossible to impregnate the resin any longer in the above-described conventional method.
To impregnate the resin into a thick reinforcing fiber substrate, it is considered to impregnate the resin into the reinforcing fiber substrate from both the resin distribution media on both surfaces of the reinforcing fiber substrate. In the above-described molding method, however, because the resin distribution media having substantially the same shape and property are disposed on both surfaces, when merely the resin is impregnated from both surface sides, the resin is impregnated in the thickness direction of the substrate simultaneously and, in a same manner, voids are hard to be pushed out in the side directions, and the voids are likely to be enclosed in the substrate. If the voids are enclosed, it is difficult to obtain a target property of a molded product. To avoid such an enclosure of voids, the resin impregnation is carried out basically from one surface side.
Further, as another problem on the above-described molding method, it is difficult to obtain good flatness on the design surface of the molded product. Namely, the above-described resin distribution medium is formed as a member having a relatively large surface irregularity with a low resistance for gas permeation to increase the resin distribution property. However, if the resin distribution media having such a relatively large surface irregularity are disposed on both surfaces of the reinforcing fiber substrate and the molding is carried out in this condition, the relatively large surface irregularity of the resin distribution medium is reflected to the design surface which is one surface of the molded product. As a result, the designability is damaged and an irregularity is formed on the surface of the molded product. Therefore, there is a problem that the properties such as aerodynamic property decrease.
To solve such problems, it is considered to use a resin distribution medium having a small surface irregularity, but if done so, the resistance for gas permeation becomes too great and, a target resin distribution property cannot be obtained. Further, because gas permeation from the reinforcing fiber substrate at the time of evacuation also deteriorates, the vacuum degree does not increase and it becomes difficult to completely impregnate the resin in the thickness direction particularly for a thick substrate.
Thus, although the degree of the irregularity of the resin distribution medium affects the resin distribution and gas permeation performances, the irregularity of the resin distribution medium for improving resin distribution and gas permeation performances (a relatively large irregularity) and the irregularity of the resin distribution medium for improving the surface property of the molded product (a relatively small irregularity) are in a relationship opposite to each other. Therefore, in a conventional method wherein substantially the same resin distribution media are disposed on both surfaces of a reinforcing fiber substrate, it is difficult to achieve both an increase in the resin impregnation property and improvement of the surface property of a molded product and it becomes particularly difficult in the molding using a thick reinforcing fiber substrate.
To improve quality on the surface property of a molded product, a condition is frequently employed wherein a gas permeable material is not disposed on a tool surface side. In such a case, however, because the gas permeation in the substrate deteriorates and the vacuum degree does not increase, reduction in impregnation may occur.
In the method of RTM molding disclosed in U.S. Pat. No. 5,052,906, a reinforcing fiber substrate is placed on a mold and a resin distribution medium is disposed at a position opposite the mold, a resin injection port and an evacuation port are disposed, they are covered with a bag material from the upper side, and a matrix resin is injected where the inside of a cavity is reduced in pressure by evacuation. The flowing-in route of the resin into the substrate is formed as a route in which the resin is distributed mainly from the injection port along the surface direction of the substrate disposed in the mold and the distributed resin is impregnated in the thickness direction of the substrate.
In the above-described method, when the resin injection is carried out at a so-called “high Vf” condition where the fiber volume content (Vf) of the reinforcing fiber substrate is 55% or more, namely, at a condition where a gap between reinforcing fibers is small, although the fiber volume content of a final molded product itself becomes high, the impregnation property of the resin into the molded product is poor. Therefore, in the case of a thick molded product having a plate thickness of, for example, 25 mm or more, the resin does not reach the corners of the molded product and only a product having a defect due to a resin non-impregnated portion is produced as a structural material.
On the other hand, in the case where the Vf of the reinforcing fibers is, for example, 45% and the gap between reinforcing fibers is relatively great, because the fiber volume content of the final molded product becomes low although a good resin impregnation property can be obtained, only a product having poor strength and lightweight property can be obtained. Namely, the resin impregnation property and the fiber volume content Vf are in a relationship opposite each other and it is difficult to achieve both improvement of the resin impregnation property and increase the fiber volume content together. Moreover, although it is preferred depending on a molded product to control the fiber volume content from the necessity of stabilization of the quality, it is also difficult to satisfy such a requirement.
Further, although usually the reinforcing fiber substrate is formed as a laminate of a plurality of reinforcing fiber materials to obtain an FRP molded product with a predetermined thickness, as to the thickness direction, that is, as to the direction perpendicular to the lamination surface in the reinforcing fiber material laminate, the resistance against the resin flow is generally high and there is a limit in the reaching distance of the resin being impregnated in the thickness direction of the substrate. Therefore, in a case where it is required to increase the number of lamination of the reinforcing fiber materials such as a case of planning to mold a high-strength product, it may be difficult to completely impregnate the resin into the corners of the reinforcing fiber material laminate and, consequently, an FRP structural material having a certain thickness or more substantially cannot be molded.
Accordingly, there is a need to provide a method of RTM molding (method of producing an FRP) which can mold even a thick FRP structural material with a good resin impregnation property, and which can realize improvement of the surface quality, increase of the lightweight property and achievement of an excellent strength.